Rice is a short-day plant, meaning it flowers (exposes its panicle (head)) when day-length becomes short. Photoreceptors that sense the day-length are called phytochrome (phy), pigment binding molecules. In rice there are three phytochrome encoding genes, PHYA, PHYB, and PHYC (Kay, S. A. et al., Nucleic Acids Res. 17: 2865-2866, 1989, Dehesh, K. et al., Mol. Gen. Genet. 225: 305-313, 1991, Tahir, M. et al., Plant Physiol. 118: 1535, 1998).
As a phytochrome mutant in monocots such as rice, a phyB mutant (ma3R) has been isolated from sorghum (Childs, K. L. et al., Plant Physiol. 113: 611-619, 1997). The ma3R mutant shows the early-flowering phenotype as well as characteristic phenotypes such as reduced chlorophyll content, stem elongation, and acceleration of apical dominance, which are obviously different from the normal plant type. Recently, the present inventors isolated phyA mutants from rice and analyzed their detailed phenotypes. As a result, no significant difference was observed in the flowering time of the mutant rice as compared to the control rice, Nipponbare, under either long-day or short-day photoperiodic conditions (Takano, M. et al., Plant Cell 13: 521-534, 2001). The se5 mutant of rice, in which levels of all phytochromes are reduced to below detectable levels, showed an early-flowering phenotype regardless of day-length photoperiod conditions (Izawa, T. et al., Plant J. 22: 391-339, 2000). However, in this mutant, all phytochrome genes, PHYA, PHYB, and PHYC, were unaltered though a mutation was found in the plastid heme oxidase encoding gene (Izawa, T. et al., Plant J. 22: 391-339, 2000).
Thus far, there has been no report of the isolation of a phyC mutant, even from the well-known experimental model plant Arabidopsis thaliana, and no report of a functional analysis of PHYC gene in relation to plant flowering time.